Voice communications are mainly used in current mobile communications. A voice signal generated by a person can be expressed as an electrical analog signal. A wired telephone transmits the analog signal, and reproduces the transmitted electrical analog signal into a voice signal.
With recent development of information technology, a method capable of more flexibly transmitting more information than an existing analog system for transmitting an electrical analog signal has been studied. As a result, a voice signal has been changed from analog to digital. A digital voice signal requires a broader bandwidth for transmission than an analog voice signal, but has a lot of merits in a lot of points such as signal transmission, flexibility, security, and cooperation with other systems. Voice compression techniques have been developed in order to complementing the disadvantage of a broad bandwidth in a digital voice signal. The change of a voice signal from analog to digital has been accelerated by the voice compression techniques, which occupy an important part of information communications.
Audio codecs can be classified into a middle-rate or low-rate codec of 16 kbps or less and a high-rate codec depending on a method of modeling a signal in compressing a voice signal. The high-rate codec uses a waveform coding system to compress a voice signal in consideration of how accurately a receiving party reconstructs an original signal. A codec enabling such a coding system is referred to as a waveform coder. On the other hand, the middle-rate or low-rate codec uses a source coding system to compress a voice signal, because the number of bits expressing an original signal decreases. The receiving party codes the voice signal using a voice signal generation model in consideration of how similar to an original signal. A coder employing such a coding system is referred to as a vocoder.